Sahih Adab al-Mufrad

It is narrated from Sayyiduna Abdullah bin Amr (may Allah be pleased with him) that he said: Sayyiduna Abu Bakr (may Allah be pleased with him) asked the Prophet (peace and blessings of Allah be upon him), "Tell me a supplication that I may say in my prayer." He said: Say, «اللهم إني ظلمت نفسي ظلماً كثيراً ، ولا يغفر الذنوب إلا أنت ، فاغفر لي من عندك مغفرة ، إنك أنت الغفور الرحيم» "I have wronged myself greatly, and none forgives sins except You, so forgive me with a complete forgiveness, for You are the Most Forgiving, Most Merciful."